ENTERPRISE JAVABEANS (EJB) technology is a well-known server-side component architecture for the JAVA 2 Platform, Enterprise Edition (J2EE) platform. EJB technology enables rapid and simplified development of distributed, transactional, secure and portable applications based on JAVA technology.
An EJB is an object that contains business logic for an application. An EJB container manages one or more EJB's contained within it. For example, for each EJB, the EJB container containing that EJB may register the EJB object, provide a remote interface for the EJB object, create and destroy EJB object instances, check security for the EJB object, manage the active state for the EJB object, and coordinate distributed transactions with the EJB object. The EJB container can also manage all persistent data within the EJB object.
An application server is a server program in a computer, within a distributed network, on which the business logic of an application executes. A cluster of interoperable application server nodes (within J2EE, for example) can provide an infrastructure (typically, transparent) for executing an application, with properties of high availability and scalability.
For example, FIG. 1 illustrates an example of a conventional architecture including a cluster 102 of application server nodes 104N1, 104N2, 104N3 and 104N4. In the FIG. 1 example, a load balancer 106 provides the capability to associate a client request (via the firewall 108) with an object in a particular application server node (generically, 104) of the cluster 102. Reference numeral 110 denotes the Internet or other sources (typically, “untrusted”) of client requests. The cluster 102 of application server nodes 104 may, for example, operate to collectively provide services such as connections to shared resources 112 (which may be, as just some examples, a file system, a database or an enterprise messaging product such as a JMS application).
Each application server node may maintain timing information to be used by business logic of containers residing on that application server node. As an example, particular business logic may be scheduled, using the timing information, to occur at regular intervals. For example, each timer may have a “time” associated with it. The time may, for example, represent an absolute time, such as a callback time of 6:00 PM daily. As another example, the time may represent an interval, such as might be used to cause a callback every 5 minutes when a particular business method is invoked for the first time. Some of the timing information may be start time, next expiry time and whether the timer is a one-time timer or an interval timer (i.e., that exists until closed by a user). Conventionally, when timing information is created by an application server node of a cluster, the timing information is made persistent in secondary storage, such as in the shared resources 112.
We now discuss high availability and failover, and the relationship of these concepts to timing information. High availability refers to the availability of resources in a computer system, even where components in the computer system have failed. In some examples of application server node and cluster configurations, the property of high availability is accomplished using a load balancing algorithm that provides for response to requests for a service as long as the system is generally operational. Typically, in normal operation, a high availability algorithm (executed by the load balancer 106, for example) forwards particular requests to corresponding preferred ones of the application server nodes 104.
When/if the preferred application server node 104 becomes unavailable, a “cluster service” notifies the components of the system (e.g., the other application server nodes) of the failure. In general, a “cluster service” is a software component that controls all aspects of server cluster operation and manages the cluster database. The high availability algorithm operates to redirect the particular requests to another one of the application server nodes 104 of the cluster 102 that is still operational. As part of the redirection of particular requests, other application server nodes 104 are configured to be able to service the particular requests previously forwarded to the now-unavailable application server node.
More generally than “high availability,” “failover” refers to the ability to perform redirection of requests with minimal disruption of service. Since timers are generally invoked in a transaction and security context, this adds complexity to considerations of failover and high availability. Handling timer expiry is a primary complexity in timer clustering. During failover, the timing information maintained by a now-unavailable application server node is made available to the other application server node, to which the requests previously forwarded to the now-unavailable server node are redirected.
In particular, typically, the timer information is persisted (i.e., stored in a way that timer information for an application server node can be recovered if the application server node fails) in a database or database cluster serving a cluster of application server nodes. During failover, the timing information is recovered by the other application server node from the database or database cluster. In this way, the business logic of other application server nodes can use the timing information if called upon to handle requests that were previously forwarded to a non-unavailable application server node.